frames that are meant to be used with a sewing machine (which ever type) roll the quilt forward to the new quilt location you never have to take the quilt off and flip.
when you're ready to work the next section you just loosen the knobs and roll the entire quilt sandwich to the next section and tighten the knobs again. you can even do this in the needle down position so you can have continuous sewing.
as far as a sewing machine for quilting - i really think this is driven by budget first and then you shop for setups that offer the most bang for your budget.
the bigger the throat the better but that also drives up the price.
there are a few setups in each price range starting as low as $1500 and as high as $17,000.
this is why its easier to choose a price range and then test drive those 3 or 4 setups that fit your range.